1. Field of the Invention
This invention relates to wellbore armored logging electric cables, as well as methods of manufacturing and using such cables. In one aspect, the invention relates to compression, stretch, and crush resistant cables which are dispatched into wellbores used with devices to analyze geologic formations adjacent a well before completion and methods of using same.
2. Description of the Related Art
Generally, geologic formations within the earth that contain oil and/or petroleum gas have properties that may be linked with the ability of the formations to contain such products. For example, formations that contain oil or petroleum gas have higher electrical resistivity than those that contain water. Formations generally comprising sandstone or limestone may contain oil or petroleum gas. Formations generally comprising shale, which may also encapsulate oil-bearing formations, may have porosities much greater than that of sandstone or limestone, but, because the grain size of shale is very small, it may be very difficult to remove the oil or gas trapped therein. Accordingly, it may be desirable to measure various characteristics of the geologic formations adjacent to a well before completion to help in determining the location of an oil- and/or petroleum gas-bearing formation as well as the amount of oil and/or petroleum gas trapped within the formation.
Logging tools, which are generally long, pipe-shaped devices, may be lowered into the well to measure such characteristics at different depths along the well. These logging tools may include gamma-ray emitters/receivers, caliper devices, resistivity-measuring devices, neutron emitters/receivers, and the like, which are used to sense characteristics of the formations adjacent the well. A wireline armored logging cable connects the logging tool with one or more electrical power sources and data analysis equipment at the earth's surface, as well as providing structural support to the logging tools as they are lowered and raised through the well. Generally, the wireline cable is spooled out of a drum unit from a truck or an offshore set up, over pulleys, and down into the well.
Wireline cables are typically formed from a combination of metallic conductors, insulative material, filler materials, jackets, and armor wires. The jackets usually encase a cable core, in which the core contains metallic conductors, insulative material, filler materials, and the like. Armor wires usually surround the jackets and core. The insulated conductors are typically placed at or near the core. Commonly, the useful life of a wellbore electric cable is typically limited to only about 6 to 24 months. In the downhole environment, wireline cables are subject to pressures that can exceed 25,000 psi and temperatures in excess of 450° F. At such high pressures, insulating material on conductors can creep due to the high compression force, leading to potential conductor failure. Also, in typical wireline cable construction, cotton yarns are cabled into the interstitial spaces between the conductors to expedite the cable core assembly process and provide a close to cylindrical surface to permit easy extrusions or helical laying of metallic wires, although these yarns are compressible as well. When a typical cable is placed under high compressive forces, the yarn compresses and contributes to deformation of the cable core containing the insulated conductors.
Commonly, polymeric jackets are placed over the cores of wireline cables. These polymeric jackets protect the core and the electrical transmittance media from the hostile chemical environment that the wireline logging cables encounter during deployment. Under high hydrostatic pressures and tension, the jacket material potentially creeps into spaces formed between the armor wires, and between the armor wires and cable core, and does not return to its original shape or position. After the cable is retrieved from the wellbore, the core becomes permanently deformed, and the insulation on helical conductors may creep into the armor wires, significantly diminishing, or eliminating, the electrical transmittance capability of the cable. Also, as the cable becomes deformed, it may also be more prone to damage from crushing as the cable, for instance, is dispatched from the spool into the wellbore over a sheave or at crossover points on the drum at high tension.
In cases were wireline cables deform when the wireline cable is bent under tension (for example, when cables go over sheaves, at crossover points on drums, or in deviated wells), the cable is compressed into an oval shape. The cable core undergoes a similar deformation and core materials can creep into gaps between the cable core and armor wires. This can lead to premature electrical shorts. Capstans are typically used in wireline applications, and can be a cause of such deformation, particularly where the normal logging tension is expected over 9,000 lbf. The capstan is necessary to lower the tension to less than 9,000 lbf and allow the cable to be taken up on the drum with out crushing the cable. The “crushing” of the cable core can occur at crossover points on the Capstan drum during such high-tension spooling. Also, the inner and outer armor layers upon applying tension and slacking and when cable is bend sharply on sheaves, drums or at cross over points on drum, can move and rotate with respect to one another resulting in the armor opening up too much. This produces enough gaps for the polymer insulated conductors to creep and fail.
Protection against cable compression damage is typically achieved by minimizing space in the core between insulated conductors using filler materials. Unfortunately, these design approaches still result in cables which are prone to compression damage, as most compression damage is still related to the performance of cotton yarn and highly flowable polymeric jacket materials. Compression and tension forces coupled with weakness of the yarn and/or polymeric jacket material may result in flow of the filler material, and thus cable deformation.
Thus, a need exists for wellbore electric cables that are resistant to compression, stretch, and crush damage as well as being resistant to material creep at both elevated temperatures and pressures. An electrical cable that can overcome one or more of the problems detailed above while conducting larger amounts of power with significant data signal transmission capability would be highly desirable, and the need is met at least in part by the following invention.